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Home - Education Resources - NDT Course Material - Ultrasonics

Introduction to Ultrasonic Testing

IntroductionBasic Principles

HistoryPresent State

Future Direction

Physics of UltrasoundWave Propagation

Modes of Sound WavesProperties of Plane Waves

Wavelength/Flaw DetectionElastic Properties of Solids

AttenuationAcoustic ImpedanceReflection/Transmission

Refraction & Snell's LawMode Conversion

Signal-to-noise Ratio

Wave Interference

Equipment & TransducersPiezoelectric TransducersCharacteristics of PT

Radiated Fields

Transducer Beam SpreadTransducer Types

Transducer Testing ITransducer Testing II

Transducer Modeling

CouplantEMATs- Lamb Wave Generationwith EMATs

- Shear Wave Generation with EMATs

- Velocity Measurementswith EMATs-Texture Measurement I with EMATs- Texture Measurement II with EMATs

- Stress Measurement with EMATs- Composite inspection with EMATs

Pulser-Receivers

Tone Burst GeneratorsFunction Generators

Impedance Matching

Data PresentationError Analysis

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Measurement TechniquesNormal Beam Inspection

Angle Beams IAngle Beams II

Crack Tip Diffraction

Automated ScanningVelocity Measurements

Measuring Attenuation

Spread SpectrumSignal Processing

Flaw Reconstruction

Calibration MethodsCalibration MethodsDAC Curves

Curvature CorrectionThompson-Gray Model

UTSIM

Grain Noise Modeling

References/Standards

Selected ApplicationsRail Inspection

Weldments

Formulae and TablesUT Material PropertiesReferences

Quizzes20 Question Quiz35 Question Quiz

50 Question Quiz

Note: These quizzes draw from the same

database of questions. Each time a quizis opened, a new set of questions will be

produced. The Collaboration for NDE

Education does not record the names of

individuals taking a quiz or the results ofa quiz.

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Home - Education Resources - NDT Course Material - Ultrasound

-Introduction to

Ultrasonic Testing

Introduction

Basic PrinciplesHistoryPresent State

Future Direction

Physics of UltrasoundWave PropagationModes of Sound Waves

Properties of Plane WavesWavelength/Flaw DetectionElastic Properties of Solids


Refraction & Snell's LawMode ConversionSignal-to-noise Ratio

Wave Interference

Equipment & TransducersPiezoelectric Transducers

Characteristics of PTRadiated FieldsTransducer Beam Spread

Transducer TypesTransducer Testing ITransducer Testing II

Transducer ModelingCouplantEMATs

Pulser-Receivers


Impedance MatchingData PresentationError Analysis


Angle Beams IAngle Beams IICrack Tip Diffraction


Measuring AttenuationSpread SpectrumSignal Processing

Flaw Reconstruction


Curvature CorrectionThompson-Gray ModelUTSIM

Grain Noise ModelingReferences/Standards

Selected Applications

Rail InspectionWeldments

Basic Principles of Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations andmake measurements. Ultrasonic inspection can be used for flaw detection/evaluation,dimensional measurements, material characterization, and more. To illustrate the generalinspection principle, a typical pulse/echo inspection configuration as illustrated belowwill be used.

A typical UT inspection system consists of several functional units, such as thepulser/receiver, transducer, and display devices. A pulser/receiver is an electronic devicethat can produce high voltage electrical pulses. Driven by the pulser, the transducer

generates high frequency ultrasonic energy. The sound energy is introduced andpropagates through the materials in the form of waves. When there is a discontinuity(such as a crack) in the wave path, part of the energy will be reflected back from the flawsurface. The reflected wave signal is transformed into an electrical signal by thetransducer and is displayed on a screen. In the applet below, the reflected signal strengthis displayed versus the time from signal generation to when a echo was received. Signaltravel time can be directly related to the distance that the signal traveled. From the signal,information about the reflector location, size, orientation and other features cansometimes be gained.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantagesof ultrasonic inspection that are often cited include:

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It is sensitive to both surface and subsurface discontinuities. The depth of penetration for flaw detection or measurement is superior to other

NDT methods. Only single-sided access is needed when the pulse-echo technique is used. It is highly accurate in determining reflector position and estimating size and

shape. Minimal part preparation is required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods. It normally requires a coupling medium to promote the transfer of sound energy

into the test specimen. Materials that are rough, irregular in shape, very small, exceptionally thin or not

homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect due to low

sound transmission and high signal noise. Linear defects oriented parallel to the sound beam may go undetected. Reference standards are required for both equipment calibration and the

characterization of flaws.

The above introduction provides a simplified introduction to the NDT method ofultrasonic testing. However, to effectively perform an inspection using ultrasonics, muchmore about the method needs to be known. The following pages present information onthe science involved in ultrasonic inspection, the equipment that is commonly used, someof the measurement techniques used, as well as other information.

Reference MaterialUT Material PropertiesReferences

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-Introduction to

Ultrasonic Testing

Introduction


Future Direction





Wave Interference





Pulser-Receivers







Flaw Reconstruction






History of Ultrasonics

Prior to World War II, sonar, the technique of sending sound waves through water andobserving the returning echoes to characterize submerged objects, inspired earlyultrasound investigators to explore ways to apply the concept to medical diagnosis. In1929 and 1935, Sokolov studied the use of ultrasonic waves in detecting metal objects.Mulhauser, in 1931, obtained a patent for using ultrasonic waves, using two transducersto detect flaws in solids. Firestone (1940) and Simons (1945) developed pulsed ultrasonictesting using a pulse-echo technique.

Shortly after the close of World War II, researchers in Japan began to explore the medical

diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-modepresentation with blips on an oscilloscope screen. That was followed by a B-modepresentation with a two dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United States and Europe untilthe 1950s. Researchers then presented their findings on the use of ultrasound to detectgallstones, breast masses, and tumors to the international medical community. Japan wasalso the first country to apply Doppler ultrasound, an application of ultrasound thatdetects internal moving objects such as blood coursing through the heart forcardiovascular investigation.

Ultrasound pioneers working in the United

States contributed many innovations andimportant discoveries to the field during thefollowing decades. Researchers learned to useultrasound to detect potential cancer and tovisualize tumors in living subjects and inexcised tissue. Real-time imaging, anothersignificant diagnostic tool for physicians,

presented ultrasound images directly on thesystem's CRT screen at the time of scanning.The introduction of spectral Doppler and latercolor Doppler depicted blood flow in variouscolors to indicate the speed and direction of the flow..

The United States also produced the earliest hand held "contact" scanner for clinical use,the second generation of B-mode equipment, and the prototype for the first articulated-arm hand held scanner, with 2-D images.

Beginnings of Nondestructive Evaluation (NDE)

Nondestructive testing has been practiced for many decades, with initial rapiddevelopments in instrumentation spurred by the technological advances that occurredduring World War II and the subsequent defense effort. During the earlier days, the

primary purpose was the detection of defects. As a part of "safe life" design, it was

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intended that a structure should not develop macroscopic defects during its life, with thedetection of such defects being a cause for removal of the component from service. Inresponse to this need, increasingly sophisticated techniques using ultrasonics, eddycurrents, x-rays, dye penetrants, magnetic particles, and other forms of interrogatingenergy emerged.

In the early 1970's, two events occurred which caused a major change in the NDT field.

First, improvements in the technology led to the ability to detect small flaws, whichcaused more parts to be rejected even though the probability of component failure hadnot changed. However, the discipline of fracture mechanics emerged, which enabled oneto predict whether a crack of a given size will fail under a particular load when amaterial's fracture toughness properties are known. Other laws were developed to predictthe growth rate of cracks under cyclic loading (fatigue). With the advent of these tools, it

became possible to accept structures containing defects if the sizes of those defects wereknown. This formed the basis for the new philosophy of "damage tolerant" design.Components having known defects could continue in service as long as it could beestablished that those defects would not grow to a critical, failure producing size.

A new challenge was thus presented to the nondestructive testing community. Detectionwas not enough. One needed to also obtain quantitative information about flaw size to

serve as an input to fracture mechanics based predictions of remaining life. The need forquantitative information was particularly strongly in the defense and nuclear powerindustries and led to the emergence of quantitative nondestructive evaluation (QNDE) asa new engineering/research discipline. A number of research programs around the worldwere started, such as the Center for Nondestructive Evaluation at Iowa State University(growing out of a major research effort at the Rockwell International Science Center); theElectric Power Research Institute in Charlotte, North Carolina; the Fraunhofer Institutefor Nondestructive Testing in Saarbrucken, Germany; and the Nondestructive TestingCentre in Harwell, England.


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-Introduction to

Ultrasonic Testing

Introduction


Future Direction





Wave Interference





Pulser-Receivers







Flaw Reconstruction






Present State of Ultrasonics

Ultrasonic testing (UT) has beenpracticed for many decades. Initial rapiddevelopments in instrumentation spurred

by the technological advances from the1950's continue today. Through the1980's and continuing through the

present, computers have providedtechnicians with smaller and more ruggedinstruments with greater capabilities.

Thickness gauging is an exampleapplication where instruments have beenrefined make data collection easier and

better. Built-in data logging capabilities allow thousands of measurements to be recordedand eliminate the need for a "scribe." Some instruments have the capability to capturewaveforms as well as thickness readings. The waveform option allows an operator toview or review the A-scan signal of thickness measurement long after the completion ofan inspection. Also, some instruments are capable of modifying the measurement basedon the surface conditions of the material. For example, the signal from a pitted or erodedinner surface of a pipe would be treated differently than a smooth surface. This has led tomore accurate and repeatable field measurements.

Many ultrasonic flaw detectors have a trigonometric function that allows for fast andaccurate location determination of flaws when performing shear wave inspections.Cathode ray tubes, for the most part, have been replaced with LED or LCD screens.These screens, in most cases, are extremely easy to view in a wide range of ambientlighting. Bright or low light working conditions encountered by technicians have littleeffect on the technician's ability to view the screen. Screens can be adjusted for

brightness, contrast, and on some instruments even the color of the screen and signal canbe selected. Transducers can be programmed with predetermined instrument settings.The operator only has to connect the transducer and the instrument will set variables suchas frequency and probe drive.

Along with computers, motion control and robotics have contributed to the advancementof ultrasonic inspections. Early on, the advantage of a stationary platform was recognizedand used in industry. Computers can be programmed to inspect large, complex shapedcomponents, with one or multiple transducers collecting information. Automatedsystems typically consisted of an immersion tank, scanning system, and recording systemfor a printout of the scan. The immersion tank can be replaced with a squirter systems,which allows the sound to be transmitted through a water column. The resultant C-scan

provides a plan or top view of the component. Scanning of components is considerablyfaster than contact hand scanning, the coupling is much more consistent. The scaninformation is collected by a computer for evaluation, transmission to a customer, andarchiving.

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Today, quantitativetheories have beendeveloped to describe theinteraction of theinterrogating fields withflaws. Modelsincorporating the resultshave been integrated withsolid model descriptionsof real-part geometries tosimulate practical

inspections. Related tools allow NDE to be considered during the design process on anequal footing with other failure-related engineering disciplines. Quantitative descriptionsof NDE performance, such as the probability of detection (POD), have become anintegral part of statistical risk assessment. Measurement procedures initially developedfor metals have been extended to engineered materials such as composites, whereanisotropy and inhomogeneity have become important issues. The rapid advances indigitization and computing capabilities have totally changed the faces of manyinstruments and the type of algorithms that are used in processing the resulting data.High-resolution imaging systems and multiple measurement modalities for characterizing

a flaw have emerged. Interest is increasing not only in detecting, characterizing, andsizing defects, but also in characterizing the materials. Goals range from thedetermination of fundamental microstructural characteristics such as grain size, porosity,and texture (preferred grain orientation), to material properties related to such failuremechanisms as fatigue, creep, and fracture toughness. As technology continues toadvance, applications of ultrasound also advance. The high-resolution imaging systemsin the laboratory today will be tools of the technician tomorrow.


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-Introduction to

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Introduction


Future Direction





Wave Interference





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Flaw Reconstruction






Future Direction of Ultrasonic Inspection

Looking to the future, those in the field of NDE see an exciting new set of opportunities.The defense and nuclear power industries have played a major role in the emergence of

NDE. Increasing global competition has led to dramatic changes in product developmentand business cycles. At the same time, aging infrastructure, from roads to buildings andaircraft, present a new set of measurement and monitoring challenges for engineers aswell as technicians.

Among the new applications of NDE spawnedby these changes is the increased emphasis onthe use of NDE to improve the productivity ofmanufacturing processes. Quantitativenondestructive evaluation (QNDE) bothincreases the amount of information aboutfailure modes and the speed with whichinformation can be obtained and facilitates thedevelopment of in-line measurements for

process control.

The phrase, "you cannot inspect in quality, youmust build it in," exemplifies the industry's focus on avoiding the formation of flaws.

Nevertheless, manufacturing flaws will never be completely eliminated and material

damage will continue to occur in-service so continual development of flaw detection andcharacterization techniques is necessary.

Advanced simulation tools that are designed for inspectability and their integration intoquantitative strategies for life management will contribute to increase the number andtypes of engineering applications of NDE. With growth in engineering applications for

NDE, there will be a need to expand the knowledge base of technicians performing theevaluations. Advanced simulation tools used in the design for inspectability may be usedto provide technical students with a greater understanding of sound behavior in materials.UTSIM, developed at Iowa State University, provides a glimpse into what may be usedin the technical classroom as an interactive laboratory tool.

As globalization continues, companies will seek to develop, with ever increasingfrequency, uniform international practices. In the area of NDE, this trend will drive theemphasis on standards, enhanced educational offerings, and simulations that can becommunicated electronically. The coming years will be exciting as NDE will continue toemerge as a full-fledged engineering discipline.

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-Introduction to

Ultrasonic Testing

Introduction


Future Direction





Wave Interference





Pulser-Receivers







Flaw Reconstruction






Wave Propagation

Ultrasonic testing is based on time-varying deformations or vibrations in materials,which is generally referred to as acoustics. All material substances are comprised ofatoms, which may be forced into vibrational motionabout their equilibrium positions.Many different patterns of vibrational motion exist at the atomic level, however, most areirrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles thatcontain many atoms that move in unison to produce a mechanical wave. When a materialis not stressed in tension or compression beyond its elastic limit, its individual particles

perform elastic oscillations. When the particles of a medium are displaced from theirequilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic

restoring forces between particles, combined with inertia of the particles, that leads to theoscillatory motionsof the medium.

In solids, sound waves can propagate in four principle modes that are based on the waythe particles oscillate. Sound can propagate as longitudinal waves, shear waves, surfacewaves, and in thin materials as plate waves. Longitudinal and shear waves are the twomodes of propagation most widely used in ultrasonic testing. The particle movementresponsible for the propagation of longitudinal and shear waves is illustrated below.

In longitudinal waves, the oscillations occur inthe longitudinal direction or the direction ofwave propagation. Since compressional anddilational forces are active in these waves, theyare also called pressure or compressional waves.They are also sometimes called density waves

because their particle density fluctuates as theymove. Compression waves can be generated inliquids, as well as solids because the energytravels through the atomic structure by a series

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of comparison and expansion (rarefaction)movements.

In the transverse or shear wave, the particlesoscillate at a right angle or transverse to the

direction of propagation. Shear waves require anacoustically solid material for effectivepropagation, and therefore, are not effectivelypropagated in materials such as liquids or gasses.Shear waves are relatively weak when comparedto longitudinal waves. In fact, shear waves areusually generated in materials using some of theenergy from longitudinal waves.


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-Introduction to

UltrasonicTesting

IntroductionBasic PrinciplesHistory

Present StateFuture Direction

Physics of Ultrasound

Wave PropagationModes of Sound WavesProperties of Plane

WavesWavelength/Flaw

DetectionElastic Properties of

SolidsAttenuationAcoustic ImpedanceReflection/Transmission

Refraction & Snell's LawMode Conversion

Signal-to-noise RatioWave Interference

Equipment &Transducers

PiezoelectricTransducers

Characteristics of PTRadiated Fields

Transducer BeamSpreadTransducer TypesTransducer Testing I

Transducer Testing IITransducer Modeling

CouplantEMATs

Pulser-ReceiversTone Burst GeneratorsFunction Generators

Impedance MatchingData Presentation

Error Analysis

MeasurementTechniquesNormal Beam

InspectionAngle Beams I

Angle Beams IICrack Tip DiffractionAutomated Scanning

Velocity MeasurementsMeasuring AttenuationSpread Spectrum

Signal ProcessingFlaw Reconstruction

Calibration Methods

Calibration MethodsDAC CurvesCurvature Correction

Thompson-Gray ModelUTSIM

Modes of Sound Wave Propagation

In air, sound travels by the compression and rarefaction of air molecules in the direction oftravel. However, in solids, molecules can support vibrations in other directions, hence, anumber of different types of sound waves are possible. Waves can be characterized in spaceby oscillatory patterns that are capable of maintaining their shape and propagating in a stablemanner. The propagation of waves is often described in terms of what are called wavemodes.

As mentioned previously, longitudinal and transverse (shear) waves are most often used inultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or

complex vibrations of the particles make other waves possible. Some of these wave modessuch as Rayleigh and Lamb waves are also useful for ultrasonic inspection.

The table below summarizes many, but not all, of the wave modes possible in solids.

Longitudinal and transverse waves were discussed on the previous page, so let's touch onsurface and plate waves here.

Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetratingto a depth of one wavelength. Surface waves combine both a longitudinal and transversemotion to create an elliptic orbit motion as shown in the image and animation below. Themajor axis of the ellipse is perpendicular to the surface of the solid. As the depth of an

individual atom from the surface increases the width of its elliptical motion decreases. Surfacewaves are generated when a longitudinal wave intersects a surface near the second criticalangle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves areuseful because they are very sensitive to surface defects (and other surface features) and theyfollow the surface around curves. Because of this, Rayleigh waves can be used to inspectareas that other waves might have difficulty reaching.

Wave Types in Solids Particle Vibrations

Longitudinal Parallel to wave direction

Transverse (Shear) Perpendicular to wave direction

Surface - Rayleigh Elliptical orbit - symmetrical mode

Plate Wave - Lamb Component perpendicular to surface (extensional wave)

Plate Wave - Love Parallel to plane layer, perpendicular to wave direction

Stoneley (Leaky RayleighWaves)

Wave guided along interface

Sezawa Antisymmetric mode

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Plate waves are similar to surface waves except they can only be generated in materials a fewwavelengths thick. Lamb waves are the most commonly used plate waves in NDT. Lambwaves are complex vibrational waves that propagate parallel to the test surface throughout thethickness of the material. Propagation of Lamb waves depends on the density and the elasticmaterial properties of a component. They are also influenced a great deal by the testfrequency and material thickness. Lamb waves are generated at an incident angle in which theparallel component of the velocity of the wave in the source is equal to the velocity of thewave in the test material. Lamb waves will travel several meters in steel and so are useful to

scan plate, wire, and tubes.

With Lamb waves, a number of modes of particlevibration are possible, but the two most common aresymmetrical and asymmetrical. The complex motion ofthe particles is similar to the elliptical orbits for surfacewaves. Symmetrical Lamb waves move in asymmetrical fashion about the median plane of theplate. This is sometimes called the extensional modebecause the wave is stretching and compressing theplate in the wave motion direction. Wave motion in thesymmetrical mode is most efficiently produced when theexciting force is parallel to the plate. The asymmetrical

Lamb wave mode is often called the flexural modebecause a large portion of the motion moves in a normal direction to the plate, and a littlemotion occurs in the direction parallel to the plate. In this mode, the body of the plate bends asthe two surfaces move in the same direction.

The generation of waves using both piezoelectric transducers and electromagnetic acoustictransducers (EMATs) are discussed in later sections.


Selected ApplicationsRail Inspection

Weldments

Reference Material

UT Material PropertiesReferences

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-Introduction to

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Future Direction





Wave Interference





Pulser-Receivers







Flaw Reconstruction






Wavelength and Defect Detection

In ultrasonic testing, the inspector must make a decision about the frequency of thetransducer that will be used. As we learned on the previous page, changing the frequencywhen the sound velocity is fixed will result in a change in the wavelength of the sound.The wavelength of the ultrasound used has a significant effect on the probability ofdetecting a discontinuity. A general rule of thumb is that a discontinuity must be largerthan one-half the wavelength to stand a reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection todescribe a technique's ability to locate flaws. Sensitivity is the ability to locate smalldiscontinuities. Sensitivity generally increases with higher frequency (shorterwavelengths). Resolution is the ability of the system to locate discontinuities that areclose together within the material or located near the part surface. Resolution alsogenerally increases as the frequency increases.

The wave frequency can also affect the capability of an inspection in adverse ways.Therefore, selecting the optimal inspection frequency often involves maintaining a

balance between the favorable and unfavorable results of the selection. Before selectingan inspection frequency, the material's grain structure and thickness, and thediscontinuity's type, size, and probable location should be considered. As frequencyincreases, sound tends to scatter from large or course grain structure and from smallimperfections within a material. Cast materials often have coarse grains and other sound

scatters that require lower frequencies to be used for evaluations of these products.Wrought and forged products with directional and refined grain structure can usually beinspected with higher frequency transducers.

Since more things in a material are likely to scatter a portion of the sound energy athigher frequencies, the penetrating power (or the maximum depth in a material that flawscan be located) is also reduced. Frequency also has an effect on the shape of theultrasonic beam. Beam spread, or the divergence of the beam from the center axis of thetransducer, and how it is affected by frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables willalso affect the ability of ultrasound to locate defects. These include the pulse length, type

and voltage applied to the crystal, properties of the crystal, backing material, transducerdiameter, and the receiver circuitry of the instrument. These are discussed in more detailin the material on signal-to-noise ratio.

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Sound Propagation in Elastic Materials

In the previous pages, it was pointed out that soundwaves propagate due to the vibrations or oscillatorymotionsof particles within a material. An ultrasonicwave may be visualized as an infinite number ofoscillating masses or particles connected by means ofelastic springs. Each individual particle is influenced bythe motion of its nearest neighbor and both inertialandelasticrestoring forces act upon each particle.

A mass on a spring has a single resonant frequencydetermined by its spring constant kand its mass m. The spring constant is the restoringforce of a spring per unit of length. Within the elastic limit of any material, there is alinear relationship between the displacement of a particle and the force attempting torestore the particle to its equilibrium position. This linear dependency is described byHooke's Law.

In terms of the spring model, Hooke's Lawsays that the restoring force due to a spring is

proportional to the length that the spring isstretched, and acts in the opposite direction.Mathematically, Hooke's Lawis written as F

=-kx, where Fis the force, kis the springconstant, and xis the amount of particledisplacement. Hooke's law is representedgraphically it the right. Please note that thespring is applying a force to the particle that isequal and opposite to the force pulling downon the particle.

The Speed of Sound

Hooke's Law, when used along with Newton's Second Law, can explain a few thingsabout the speed of sound. The speed of sound within a material is a function of the

properties of the material and is independent of the amplitude of the sound wave.Newton's Second Law says that the force applied to a particle will be balanced by theparticle's mass and the acceleration of the the particle. Mathematically, Newton's SecondLaw is written as F = ma. Hooke's Law then says that this force will be balanced by aforce in the opposite direction that is dependent on the amount of displacement and thespring constant (F = -kx). Therefore, since the applied force and the restoring force areequal, ma = -kx can be written. The negative sign indicates that the force is in theopposite direction.

Since the mass mand the spring constant k are constants for any given material, it can beseen that the acceleration aand the displacement xare the only variables. It can also be

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seen that they are directly proportional. For instance, if the displacement of the particleincreases, so does its acceleration. It turns out that the time that it takes a particle to moveand return to its equilibrium position is independent of the force applied. So, within agiven material, sound always travels at the same speed no matter how much force isapplied when other variables, such as temperature, are held constant.

What properties of material affect its speed of sound?

Of course, sound does travel at different speeds in different materials. This is because themass of the atomic particles and the spring constants are different for different materials.The mass of the particles is related to the density of the material, and the spring constantis related to the elastic constants of a material. The general relationship between thespeed of sound in a solid and its density and elastic constants is given by the followingequation:

Where Vis the speed of sound, Cis the elastic constant, and pis the material density.This equation may take a number of different forms depending on the type of wave(longitudinal or shear) and which of the elastic constants that are used. The typical elasticconstants of a materials include:

Young's Modulus, E: a proportionality constant between uniaxial stress and strain. Poisson's Ratio, n: the ratio of radial strain to axial strain Bulk modulus, K: a measure of the incompressibility of a body subjected to

hydrostatic pressure. Shear Modulus, G: also called rigidity, a measure of a substance's resistance to

shear.

Lame's Constants, l andm: material constants that are derived from Young'sModulus and Poisson's Ratio.

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson'sRatio are commonly used. When calculating the velocity of a shear wave, the shearmodulus is used. It is often most convenient to make the calculations using Lame'sConstants, which are derived from Young's Modulus and Poisson's Ratio.

It must also be mentioned that the subscript ijattached to C in the above equation is usedto indicate the directionality of the elastic constants with respect to the wave type anddirection of wave travel. In isotropic materials, the elastic constants are the same for alldirections within the material. However, most materials are anisotropic and the elasticconstants differ with each direction. For example, in a piece of rolled aluminum plate, the

grains are elongated in one direction and compressed in the others and the elasticconstants for the longitudinal direction are different than those for the transverse or shorttransverse directions.

Examples of approximate compressional sound velocities in materials are:

Aluminum - 0.632 cm/microsecond 1020 steel - 0.589 cm/microsecond Cast iron - 0.480 cm/microsecond.


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Examples of approximate shear sound velocities in materials are:

Aluminum - 0.313 cm/microsecond 1020 steel - 0.324 cm/microsecond Cast iron - 0.240 cm/microsecond.

When comparing compressional and shear velocities, it can be noted that shear velocity

is approximately one half that of compressional velocity. The sound velocities for avariety of materials can be found in the ultrasonic properties tables in the generalresources section of this site.




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Attenuation of Sound Waves

When sound travels through a medium, its intensity diminisheswith distance. In idealized materials, sound pressure (signalamplitude) is only reduced by the spreading of the wave. Naturalmaterials, however, all produce an effect which further weakens thesound. This further weakening results from scattering andabsorption. Scattering is the reflection of the sound in directionsother than its original direction of propagation. Absorption is the conversion of thesound energy to other forms of energy. The combined effect of scattering and absorptionis calledattenuation. Ultrasonic attenuation is the decay rate of the wave as it

propagates through material.

Attenuation of sound within a material itself is often not of intrinsic interest. However,natural properties and loading conditions can be related to attenuation. Attenuation oftenserves as a measurement tool that leads to the formation of theories to explain physical orchemical phenomenon that decreases the ultrasonic intensity.

The amplitude change of a decaying plane wave can be expressed as:

In this expression A0is the unattenuated amplitude of the propagating wave at some

location. The amplitude Ais the reduced amplitude after the wave has traveled a distance

zfrom that initial location. The quantity is the attenuation coefficient of the wave

traveling in the z-direction. The dimensions of are nepers/length, where a neper is a

dimensionless quantity. The term e is the exponential (or Napier's constant) which isequal to approximately 2.71828.

The units of the attenuation value in Nepers per meter (Np/m) can be converted todecibels/length by dividing by 0.1151. Decibels is a more common unit when relating the

amplitudes of two signals.

Attenuation is generally proportional to the square of sound frequency. Quoted values ofattenuation are often given for a single frequency, or an attenuation value averaged overmany frequencies may be given. Also, the actual value of the attenuation coefficient for agiven material is highly dependent on the way in which the material was manufactured.Thus, quoted values of attenuation only give a rough indication of the attenuation andshould not be automatically trusted. Generally, a reliable value of attenuation can only beobtained by determining the attenuation experimentally for the particular material beingused.

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Attenuation can be determined by evaluating the multiple backwall reflections seen in atypical A-scan display like the one shown in the image at the top of the page. Thenumber of decibels between two adjacent signals is measured and this value is divided bythe time interval between them. This calculation produces a attenuation coefficient in

decibels per unit time Ut. This value can be converted to nepers/length by the followingequation.

Where vis the velocity of sound in meters per second and Utis in decibels per second.


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-Introduction to

Ultrasonic Testing

Introduction


Future Direction





Wave Interference





Pulser-Receivers







Flaw Reconstruction






Acoustic Impedance

Sound travels through materials under the influence of sound pressure. Becausemolecules or atoms of a solid are bound elastically to one another, the excess pressureresults in a wave propagating through the solid.

The acoustic impedance (Z) of a material is defined as the product of its density (p) andacoustic velocity (V).

Z = pV

Acoustic impedance is important in

1. the determination of acoustic transmission and reflection at the boundary of twomaterials having different acoustic impedances.

2. the design of ultrasonic transducers.3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any material, solong as its density (p) and acoustic velocity (V) are known. The applet also shows how achange in the impedance affects the amount of acoustic energy that is reflected andtransmitted. The values of the reflected and transmitted energy are the fractionalamounts of the total energy incident on the interface. Note that the fractional amount of

transmitted sound energy plus the fractional amount of reflected sound energy equalsone. The calculation used to arrive at these values will be discussed on the next page.

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Reflection and Transmission Coefficients (Pressure)

Ultrasonic waves are reflected at boundaries where there is a difference in acousticimpedances (Z) of the materials on each side of the boundary. (See preceding page formore information on acoustic impedance.) This difference in Z is commonly referred toas the impedance mismatch. The greater the impedance mismatch, the greater the

percentage of energy that will be reflected at the interface or boundary between onemedium and another.

The fraction of the incident wave intensity that is refracted can be derived becauseparticle velocity and local particle pressures must be continuous across the boundary.

When the acoustic impedances of the materials on both sides of the boundary are known,the fraction of the incident wave intensity that is reflected can be calculated with theequation below. The value produced is known as the reflection coefficient. Multiplyingthe reflection coefficient by 100 yields the amount of energy reflected as a percentage ofthe original energy.

Since the amount of reflected energy plus the transmitted energy must equal the total

amount of incident energy, the transmission coefficient is calculated by simplysubtracting the reflection coefficient from one.

Formulations for acoustic reflection and transmission coefficients (pressure) are shown inthe interactive applet below. Different materials may be selected or the material velocityand density may be altered to change the acoustic impedance of one or both materials.The red arrowrepresents reflected sound and theblue arrowrepresents transmittedsound.

Note that the reflection and transmission coefficients are often expressed in decibels (dB)

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-Introduction to

UltrasonicTesting

IntroductionBasic PrinciplesHistoryPresent StateFuture Direction

Physics ofUltrasoundWave PropagationModes of Sound WavesProperties of PlaneWavesWavelength/FlawDetection

Elastic Properties ofSolidsAttenuationAcoustic ImpedanceReflection/TransmissionRefraction & Snell'sLawMode ConversionSignal-to-noise RatioWave Interference

Equipment &TransducersPiezoelectricTransducersCharacteristics of PTRadiated FieldsTransducer Beam

SpreadTransducer TypesTransducer Testing ITransducer Testing IITransducer ModelingCouplantEMATsPulser-ReceiversTone Burst GeneratorsFunction GeneratorsImpedance MatchingData PresentationError Analysis

MeasurementTechniquesNormal BeamInspectionAngle Beams IAngle Beams IICrack Tip DiffractionAutomated ScanningVelocity MeasurementsMeasuring AttenuationSpread SpectrumSignal ProcessingFlaw Reconstruction

Calibration MethodsCalibration MethodsDAC CurvesCurvature CorrectionThompson-Gray Model

Refraction and Snell's Law

When an ultrasounic wave passes through an interface between twomaterials at an oblique angle, and the materials have different indices ofrefraction, both reflected and refracted waves are produced. This alsooccurs with light, which is why objects seen across an interface appear to

be shifted relative to where they really are. For example, if you lookstraight down at an object at the bottom of a glass of water, it looks closerthan it really is. A good way to visualize how light and sound refract is toshine a flashlight into a bowl of slightly cloudy water noting the refractionangle with respect to the incident angle.

Refraction takes place at an interface due to the different velocities of theacoustic waves within the two materials. The velocity of sound in eachmaterial is determined by the material properties (elastic modulus anddensity) for that material. In the animation below, a series of plane wavesare shown traveling in one material and entering a second material that has a higher acousticvelocity. Therefore, when the wave encounters the interface between these two materials, the

portion of the wave in the second material is moving faster than the portion of the wave in thefirst material. It can be seen that this causes the wave to bend.

Snell's Law describes the relationship between the angles and the velocities of the waves. Snell's

law equates the ratio of material velocities V1and V2to the ratio of the sine'sof incident (

)

and refracted (

) angles, as shown in the following equation.

Where:

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Note that in the diagram, there is a reflected longitudinal wave (VL

1') shown. This wave is

reflected at the same angle as the incident wave because the two waves are traveling in the samematerial, and hence have the same velocities. This reflected wave is unimportant in ourexplanation of Snell's Law, but it should be remembered that some of the wave energy isreflected at the interface. In the applet below, only the incident and refracted longitudinal wavesare shown. The angle of either wave can be adjusted by clicking and dragging the mouse in theregion of the arrows. Values for the angles or acoustic velocities can also be entered in the

dialog boxes so the that applet can be used as a Snell's Law calculator.

When a longitudinal wave moves from a slower to a faster material, there is an incident angle

that makes the angle of refraction for the wave 90o. This is know as the first critical angle. Thefirst critical angle can be found from Snell's law by putting in an angle of 90 for the angle ofthe refracted ray. At the critical angle of incidence, much of the acoustic energy is in the form ofan inhomogeneous compression wave, which travels along the interface and decaysexponentially with depth from the interface. This wave is sometimes referred to as a "creepwave." Because of their inhomogeneous nature and the fact that they decay rapidly, creep wavesare not used as extensively as Rayleigh surface waves in NDT. However, creep waves aresometimes more useful than Rayleigh waves because they suffer less from surface irregularitiesand coarse material microstructure due to their longer wavelengths.

VL

1is the longitudinal wave velocity in material

1.

VL

2is the longitudinal wave velocity in material

2.

UTSIMGrain Noise ModelingReferences/Standards

SelectedApplicationsRail InspectionWeldments

Reference Material

UT Material PropertiesReferences

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Wave Interference





Pulser-Receivers







Flaw Reconstruction






Mode Conversion

When sound travels in a solid material, one form of wave energy can be transformed intoanother form. For example, when a longitudinal waves hits an interface at an angle, someof the energy can cause particle movement in the transverse direction to start a shear(transverse) wave. Mode conversion occurs when a wave encounters an interface

between materials of different acoustic impedances and the incident angle is not normalto the interface. From the ray tracing movie below, it can be seen that since modeconversion occurs every time a wave encounters an interface at an angle, ultrasonicsignals can become confusing at times.

In the previous section, it was pointed out that when sound waves pass through an

interface between materials having different acoustic velocities, refraction takes place atthe interface. The larger the difference in acoustic velocities between the two materials,the more the sound is refracted. Notice that the shear wave is not refracted as much as thelongitudinal wave. This occurs because shear waves travel slower than longitudinalwaves. Therefore, the velocity difference between the incident longitudinal wave and theshear wave is not as great as it is between the incident and refracted longitudinal waves.Also note that when a longitudinal wave is reflected inside the material, the reflectedshear wave is reflected at a smaller angle than the reflected longitudinal wave. This isalso due to the fact that the shear velocity is less than the longitudinal velocity within agiven material.

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Snell's Law holds true for shear waves as well as longitudinal waves and can be writtenas follows.

Where:V

L1is the longitudinal wave velocity in material 1.

VL2is the longitudinal wave velocity in material 2.

VS1

is the shear wave velocity in material 1.

VS2

is the shear wave velocity in material 2.

In the applet below, the shear (transverse) wave raypath has been added. The ray paths of the waves can

be adjusted by clicking and dragging in the vicinity of the arrows. Values for the anglesor the wave velocities can also be entered into the dialog boxes. It can be seen from theapplet that when a wave moves from a slower to a faster material, there is an incidentangle which makes the angle of refraction for the longitudinal wave 90 degrees. As

mentioned on the previous page, this is known as the first critical angle and all of theenergy from the refracted longitudinal wave is now converted to a surface followinglongitudinal wave. This surface following wave is sometime referred to as a creep waveand it is not very useful in NDT because it dampens out very rapidly.

Beyond the first critical angle, only the shear wave propagates into the material. For thisreason, most angle beam transducers use a shear wave so that the signal is notcomplicated by having two waves present. In may cases there is also an incident anglethat makes the angle of refraction for the shear wave 90 degrees. This is know as thesecond critical angle and at this point, all of the wave energy is reflected or refracted intoa surface following shear wave or shear creep wave. Slightly beyond the second criticalangle, surface waves will be generated.

Note that the applet defaults to compressional velocity in the second material. Therefracted compressional wave angle will be generated for given materials and angles. Tofind the angle of incidence required to generate a shear wave at a given angle completethe following:

1. Set V1 to the longitudinal wave velocity of material 1. This material could be thetransducer wedge or the immersion liquid.

2. Set V2 to the shear wave velocity (approximately one-half its compressional


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Wave Interference





Pulser-Receivers







Flaw Reconstruction






Signal-to-Noise Ratio

In a previous page, the effect that frequency and wavelength have on flaw detectabilitywas discussed. However, the detection of a defect involves many factors other than therelationship of wavelength and flaw size. For example, the amount of sound that reflectsfrom a defect is also dependent on the acoustic impedance mismatch between the flawand the surrounding material. A void is generally a better reflector than a metallicinclusion because the impedance mismatch is greater between air and metal than betweentwo metals.

Often, the surrounding material has competing reflections. Microstructure grains in

metals and the aggregate of concrete are a couple of examples. A good measure ofdetectability of a flaw is its signal-to-noise ratio (S/N). The signal-to-noise ratio is ameasure of how the signal from the defect compares to other background reflections(categorized as "noise"). A signal-to-noise ratio of 3 to 1 is often required as a minimum.The absolute noise level and the absolute strength of an echo from a "small" defectdepends on a number of factors, which include:

The probe size and focal properties. The probe frequency, bandwidth and efficiency. The inspection path and distance (water and/or solid). The interface (surface curvature and roughness). The flaw location with respect to the incident beam.

The inherent noisiness of the metal microstructure. The inherent reflectivity of the flaw, which is dependent on its acoustic impedance,

size, shape, and orientation. Cracks and volumetric defects can reflect ultrasonic waves quite differently. Many

cracks are "invisible" from one direction and strong reflectors from another. Multifaceted flaws will tend to scatter sound away from the transducer.

The following formula relates some of the variables affecting the signal-to-noise ratio(S/N) of a defect:

Rather than go into the details of this formulation, a few fundamental relationships can bepointed out. The signal-to-noise ratio (S/N), and therefore, the detectability of a defect:

Increases with increasing flaw size (scattering amplitude). The detectability of a

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defect is directly proportional to its size. Increases with a more focused beam. In other words, flaw detectability is inversely

proportional to the transducer beam width. Increases with decreasing pulse width (delta-t). In other words, flaw detectability is

inversely proportional to the duration of the pulse produced by an ultrasonictransducer. The shorter the pulse (often higher frequency), the better the detectionof the defect. Shorter pulses correspond to broader bandwidth frequency response.See the figure below showing the waveform of a transducer and its correspondingfrequency spectrum.

Decreases in materials with high density and/or a high ultrasonic velocity. Thesignal-to-noise ratio (S/N) is inversely proportional to material density andacoustic velocity.

Generally increases with frequency. However, in some materials, such as titaniumalloys, both the "A

flaw" and the "Figure of Merit (FOM)" terms in the equation

change at about the same rate with changing frequency. So, in some cases, thesignal-to-noise ratio (S/N) can be somewhat independent of frequency.


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-Introduction to

UltrasonicTesting

IntroductionBasic PrinciplesHistoryPresent StateFuture Direction

Physics of UltrasoundWave PropagationModes of Sound WavesProperties of PlaneWavesWavelength/FlawDetectionElastic Properties ofSolidsAttenuation

Acoustic ImpedanceReflection/TransmissionRefraction & Snell's LawMode ConversionSignal-to-noise RatioWave Interference

Equipment &Transducers

Piezoelectric TransducersCharacteristics of PTRadiated Fields

Transducer Beam SpreadTransducer TypesTransducer Testing I

Transducer Testing IITransducer ModelingCouplantEMATsPulser-ReceiversTone Burst GeneratorsFunction Generators


MeasurementTechniquesNormal Beam InspectionAngle Beams I

Angle Beams IICrack Tip DiffractionAutomated Scanning

Velocity MeasurementsMeasuring AttenuationSpread SpectrumSignal ProcessingFlaw Reconstruction


Curvature CorrectionThompson-Gray ModelUTSIMGrain Noise ModelingReferences/Standards

Wave Interaction or Interference

Before we move into the next section, the subject of wave interaction must be covered sinceit is important when trying to understand the performance of an ultrasonic transducer. On theprevious pages, wave propagation was discussed as if a single sinusoidal wave waspropagating through the material. However, the sound that emanates from an ultrasonictransducer does not originate from a single point, but instead originates from many pointsalong the surface of the piezoelectric element. This results in a sound field with many wavesinteracting or interfering with each other.

When waves interact, they superimpose on each other, and the amplitude of the sound

pressure or particle displacement at any point of interaction is the sum of the amplitudes ofthe two individual waves. First, let's consider two identical waves that originate from thesame point. When they are in phase(so that the peaks and valleys of one are exactly alignedwith those of the other), they combine to double the displacement of either wave actingalone. When they are completely out of phase(so that the peaks of one wave are exactlyaligned with the valleys of the other wave), they combine to cancel each other out. When thetwo waves are not completely in phaseor out of phase, the resulting wave is the sum of thewave amplitudes for all points along the wave.

When the origins of the two interacting waves are not the same, it isa little harder to picture the wave interaction, but the principles arethe same. Up until now, we have primarily looked at waves in theform of a 2D plot of wave amplitude versus wave position.

However, anyone that has dropped something in a pool of watercan picture the waves radiating out from the source with a circularwave front. If two objects are dropped a short distance apart intothe pool of water, their waves will radiate out from their sourcesand interact with each other. At every point where the wavesinteract, the amplitude of the particle displacement is the combinedsum of the amplitudes of the particle displacement of the individualwaves.

With an ultrasonic transducer, the waves propagate out from the

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transducer face with a circular wave front. If it were possible to getthe waves to propagate out from a single point on the transducerface, the sound field would appear as shown in the upper image tothe right. Consider the light areas to be areas of rarefaction and thedark areas to be areas of compression.

However, as stated previously, sound waves originate from

multiple points along the face of the transducer. The lower image tothe right shows what the sound field would look like if the wavesoriginated from just two points. It can be seen that where the wavesinteract, there are areas of constructive and destructive interference.The points of constructive interference are often referred to asnodes. Of course, there are more than two points of origin along theface of a transducer. The image below shows five points of soundorigination. It can be seen that near the face of the transducer, thereare extensive fluctuations or nodes and the sound field is veryuneven. In ultrasonic testing, this in known as the near field (nearzone) or Fresnel zone. The sound field is more uniform away from the transducer in the farfield, or Fraunhofer zone, where the beam spreads out in a pattern originating from the centerof the transducer. It should be noted that even in the far field, it is not a uniform wave front.

However, at some distance from the face of the transducer and central to the face of thetransducer, a uniform and intense wave field develops.

The curvature and the area over which the sound is being generated, the speed that the soundwaves travel within a material and the frequency of the sound all affect the sound field. Usethe Java applet below to experiment with these variables and see how the sound field isaffected.

Selected ApplicationsRail InspectionWeldments


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Strong,uniform

sound field

Multiple points

of soundorigination alongthe face of thetransducer




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Wave Interference





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Flaw Reconstruction






Piezoelectric Transducers

The conversion of electrical pulses tomechanical vibrations and theconversion of returned mechanicalvibrations back into electrical energy isthe basis for ultrasonic testing. Theactive element is the heart of thetransducer as it converts the electricalenergy to acoustic energy, and viceversa. The active element is basically a

piece of polarized material (i.e. some parts of the molecule are positively charged, whileother parts of the molecule are negatively charged) with electrodes attached to two of itsopposite faces. When an electric field is applied across the material, the polarizedmolecules will align themselves with the electric field, resulting in induced dipoleswithin the molecular or crystal structure of the material. This alignment of molecules willcause the material to change dimensions. This phenomenon is known as electrostriction.In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate(BaTiO3) will produce an electric field when the material changes dimensions as a resultof an imposed mechanical force. This phenomenon is known as the piezoelectric effect.Additional information on why certain materials produce this effect can be found in thelinked presentation material, which was produced by the Valpey Fisher Corporation.

Piezoelectric Effect (PPT, 89kb) Piezoelectric Elements (PPT, 178kb)

The active element of most acoustic transducers usedtoday is apiezoelectricceramic, which can be cut invarious ways to produce different wave modes. A large

piezoelectric ceramic element can be seen in the imageof a sectioned low frequency transducer. Preceding theadvent of piezoelectric ceramics in the early 1950's,

piezoelectric crystals made from quartz crystals andmagnetostrictivematerials were primarily used. Theactive element is still sometimes referred to as the crystal

by old timers in the NDT field. When piezoelectricceramics were introduced, they soon became the

dominant material for transducers due to their goodpiezoelectric properties and their ease of manufactureinto a variety of shapes and sizes. They also operate at low voltage and are usable up to

about 300oC. The first piezoceramic in general use was barium titanate, and that wasfollowed during the 1960's by lead zirconate titanate compositions, which are now themost commonly employed ceramic for making transducers. New materials such as piezo-

polymers and composites are also being used in some applications.

The thickness of the active element is determined by the desired frequency of thetransducer. A thin wafer element vibrates with a wavelength that is twice its thickness.

Page 1 of 2Piezoelectric Transducers

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Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiatedwavelength. The higher the frequency of the transducer, the thinner the active element.The primary reason that high frequency contact transducers are not produced is becausethe element is very thin and too fragile.


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-Introduction to

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Characteristics of Piezoelectric Transducers

The transducer is a very important part of the ultrasonic instrumentation system. Asdiscussed on the previous page, the transducer incorporates a piezoelectric element,which converts electrical signals into mechanical vibrations (transmit mode) andmechanical vibrations into electrical signals (receive mode). Many factors, includingmaterial, mechanical and electrical construction, and the external mechanical andelectrical load conditions, influence the behavior of a transducer. Mechanicalconstruction includes parameters such as the radiation surface area, mechanical damping,housing, connector type and other variables of physical construction. As of this writing,transducer manufacturers are hard pressed when constructing two transducers that have

identical performance characteristics.

A cut away of a typical contact transducer is shownabove. It was previously learned that the piezoelectricelement is cut to 1/2 the desired wavelength. To get asmuch energy out of the transducer as possible, animpedance matching is placed between the activeelement and the face of the transducer. Optimalimpedance matching is achieved by sizing thematching layer so that its thickness is 1/4 of thedesired wavelength. This keeps waves that werereflected within the matching layer in phase whenthey exit the layer (as illustrated in the image to theright). For contact transducers, the matching layer ismade from a material that has an acoustical impedance between the active element andsteel. Immersion transducers have a matching layer with an acoustical impedance

between the active element and water. Contact transducers also incorporate a wear plateto protect the matching layer and active element from scratching.

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The backing material supporting the crystal has a great influence on the dampingcharacteristics of a transducer. Using a backing material with an impedance similar tothat of the active element will produce the most effective damping. Such a transducerwill have a wider bandwidth resulting in higher sensitivity. As the mismatch inimpedance between the active element and the backing material increases, material

penetration increases but transducer sensitivity is reduced.

Transducer Efficiency, Bandwidth and Frequency

Some transducers are specially fabricated to be more efficient transmitters and others tobe more efficient receivers. A transducer that performs well in one application will notalways produce the desired results in a different application. For example, sensitivity tosmall defects is proportional to the product of the efficiency of the transducer as atransmitter and a receiver. Resolution, the ability to locate defects near the surface or inclose proximity in the material, requires a highly damped transducer.

It is also important to understand the concept of bandwidth, or range of frequencies,associated with a transducer. The frequency noted on a transducer is the central or centerfrequency and depends primarily on the backing material. Highly damped transducerswill respond to frequencies above and below the central frequency. The broad frequency

range provides a transducer with high resolving power. Less damped transducers willexhibit a narrower frequency range and poorer resolving power, but greater penetration.The central frequency will also define the capabilities of a transducer. Lower frequencies(0.5MHz-2.25MHz) provide greater energy and penetration in a material, while highfrequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greatersensitivity to small discontinuities. High frequency transducers, when used with the

proper instrumentation, can improve flaw resolution and thickness measurementcapabilities dramatically. Broadband transducers with frequencies up to 150 MHz arecommercially available.

Transducers are constructed to withstand some abuse, but they should be handledcarefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the

backing material. Damage to a transducer is often noted on the A-scan presentation as anenlargement of the initial pulse.


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